Japanese Laid-Open Patent Publication No. 2011-91887 describes an example of a motor yoke housing including a tubular case and a fastening flange. The tubular case includes a closed end and an open end, and the fastening flange is formed on the open end of the tubular case. A field magnet and an armature are arranged in the tubular case. A drawing process is performed to form the yoke housing from a metal sheet. Then, a pressing process is performed to press out the flange from the metal sheet.
A motor used in, for example, a power window device of a vehicle, needs to be thin (narrow) in a direction orthogonal to the axis of the motor due to the limitation in the available space. As shown in FIG. 11A, a yoke housing 101 may use a flange 102 that is narrowed, or generally rectangular, as viewed in the axial direction to reduce the size of the motor in the direction orthogonal to the motor axis. Further, as shown in FIG. 11A, to reduce the size of the motor, it is desirable that the length of the short sides of the flange 102 be equal to the outer diameter of a tubular case 103. In other words, it is desirable that the long sides of the flange 102 arranged on the outer circumferential surface of the tubular case 103 as viewed in the axial direction.
As shown in FIG. 11A, when the length of the short sides of the flange 102 is equal to the outer diameter of the tubular case 103, there is an interval between the outer circumferential surface of the tubular case 103 and each longitudinal edge of the flange 102. However, there is no interval between the outer circumferential surface of the tubular case 103 and the each lateral edge of the flange 102. Thus, when manufacturing the yoke housing 101, the removal of the flange 102 from a metal sheet 104 during a pressing process results in residual stress, which was generated when drawing the tubular case 103, acting in the lateral direction in which the flange 102 does not extend. Consequently, as shown in FIG. 11B, the open end of the tubular case 103 may deform so as to widen in the lateral direction of the flange 102.